The micro scale is not specifically defined, but, used herein, it means employing dimensions between 1 and 1000 microns. The micro scale is of interest in many environments, particularly in the analytical field where possibly only small quantities of reactants are available.
Plasmas are gases to which an electric field is applied, dissociating molecules of the gas into charged ions. The ions can be employed for a number of purposes although it is a well-recognised property of plasmas that ions are extinguished once they collide with surfaces, for example the surfaces of the container in which the plasma is formed, or the plates of the electrodes. Consequently, containers tend to be large, and plates separated by as large a distance as possible, so that the energy put in to create ions is not lost by ion extinction.
Such large distances have the corollary effect that electric voltages applied by the plates have to be substantial in order to create a sufficiently concentrated electric field to form the desired plasma. Indeed, the high voltage required is frequently the reason why plasmas are not employed in many situations. For example, ozone is produced in plasma, but the cost of production using this technique renders it uneconomic for many purposes, particularly sewage treatment and water sterilisation, which is a relatively low value operation.
Ozone is known to be useful in two applications, at least. A first application is in the sterilisation of water for drinking purposes. Indeed, ozone is the preferred material for water sterilisation in many countries, but not in the UK where economic considerations take precedence. Thus, the less expensive chlorination process is used in the UK to obtain potable water. However, the improved taste of water sterilised with ozone could be achieved if a cost effective method of ozone production was available. This would be especially true if an efficient mechanism for distributing the ozone within the water was also available.
A second application is in a reactor. Ozone is employed to break down unsaturated carbon-carbon bonds of hydrocarbons. This splits large molecules into smaller ones. The smaller molecules can be identified and quantified by other techniques (such as mass spectrometry and chromatography) and this is potentially identifying of the original larger molecule.
There are also many uses of ozone that are economic. For example, ozone is used in the field of medical sterilizers and gas phase atmospheric “cleansers” (the latter removing bacteria, dust and dust mites). These applications use large plasma reactor chambers that are energy intensive.
A method of generating ozone with less power requirements would be desirable. In the field of water treatment, if an economic method of ozone production could be developed, it would also be desirable to have an effective delivery method for the ozone into the water being treated.
Hydrogen is also a useful gas. It can be produced by the plasmolysis of steam. However, the energy employed in its production must not be so high that a good proportion of it is not recoverable later in a hydrogen fuel cell, for instance. Indeed, it is well recognised that production of hydrogen using electricity generated from solar power or other “free” sources is a means of “storing” the electricity for later use, for example when it is wanted but when the solar or wind power have temporarily subsided. It would be desirable to have an efficient production method for hydrogen, as well as a simple separation mechanism to separate it from oxygen, its co-product of plasmolysis of steam.
Microplasma reactors are known [1,2]. Agiral et al [1] concerns reactors to decompose CO2 and relates to the incorporation of nanotubes and nanowires as electrodes. Lindner et al [2] employs microplasmas as a means of reforming hydrocarbons, in particular JP-8 jet fuel, to produce hydrogen for solid oxide fuel cells. However, neither of these papers actually discuss the economics of the proposals.